Shape selective hydrocarbon conversion over pre-selectivated, activated catalyst

ABSTRACT

A process for a shape selective hydrocarbon conversion such as toluene disproportionation involves contacting a reaction stream under conversion conditions with a catalytic molecular sieve which has been pre-selectivated and concurrently activated by contact with a substantially aqueous solution of an organosilicon compound. The invention also includes a method for concurrently preselectivating and activating a catalyst and the shape selectivated, activated catalyst which results from this method.

This is a division of Ser. No. 022,205, filed Feb. 25, 1993, now U.S. Pat. No. 5,349,113.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to copending U.S. patent applications Ser. Nos. 850,104 and 850,105 both filed Mar. 12, 1992 which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a process for shape selective hydrocarbon conversions such as the regioselective production of para-substituted compounds, e.g. para-xylene. The invention is also directed to a modified catalyst and method of modifying the catalyst. A catalytic molecular sieve is modified by treatment with an aqueous solution of a water soluble organosilicon compound. The catalytic activity and selectivity of the molecular sieve are both increased by the modification treatment.

2. Description of the Prior Art

The term shape-selective catalysis describes unexpected catalytic selectivities in zeolites. The principles behind shape selective catalysis have been reviewed extensively, e.g. by N. Y. Chen, W. E. Garwood and F. G. Dwyer, "Shape Selective Catalysis in Industrial Applications," 36, Marcel Dekker, Inc. (1989). Within a zeolite pore, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, oligomerization and aromatic disproportionation, alkylation or transalkylation reactions are governed by constraints imposed by the channel size. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the zeolite pores to react; while product selectivity occurs when some of the products cannot leave the zeolite channels. Product distributions can also be altered by transition state selectivity in which certain reactions cannot occur because the reaction transition state is too large to form within the zeolite pores or cages. Another type of selectivity results from configurational diffusion where the dimensions of the molecule approach that of the zeolite pore system. A small change in dimensions of the molecule or the zeolite pore can result in large diffusion changes leading to different product distributions. This type of shape selective catalysis is demonstrated, for example, in toluene selective disproportionation to p-xylene.

The synthesis of para-xylene is typically performed by methylation of toluene over a catalyst under conversion conditions. Examples are the reaction of toluene with methanol as described by Chen et al., J. Amer. Chem. Sec. 1979, 101, 6783, and toluene disproportionation, as described by Pines in "The Chemistry of Catalytic Hydrocarbon Conversions", Academic Press, New York, 1981, p. 72 Such methods typically result in the production of a mixture including para-xylene, ortho-xylene, and meta-xylene. Depending upon the para-selectivity of the catalyst and the reaction conditions, different percentages of para-xylene are obtained. The yield, i.e., the amount of feedstock actually converted to xylene, is also affected by the catalyst and the reaction conditions.

Previously known toluene methylation reactions typically provide many by-products such as those indicated in the following formula:

Thermodynamic Equilibria for Toluene Conversion to the Products Indicated

Non-MTPX ##STR1##

One method for increasing para-selectivity of zeolite catalysts is to modify the catalyst by treatment with "selectivating agents". Various silicon compounds have been used to modify catalysts to improve selectivity in hydrocarbon conversion processes. For example, U.S. Pat. Nos. 4,145,315, 4,127,616 and 4,090,981, describe the use of a silicone compound dissolved in an organic solvent to treat a zeolite. U.S. Pat. Nos. 4,465,886 and 4,477,583 describe the use of an aqueous emulsion of a silicone to treat a zeolite. U.S. Pat. Nos. 4,950,835 and 4,927,979 describe the use of alkoxysilanes carried by gases or organic solvents to treat a zeolite. U.S. Pat. Nos. 4,100,215 and 3,698,157 describe the use of silanes in hydrocarbons, e.g., pyridine or ethers, to treat a zeolite.

Some catalyst modification procedures, for example, U.S. Pat. Nos. 4,477,583 and 4,127,616 have been successful in obtaining para-selectivity, i.e., para-xylene/all xylenes, of greater than about 90% but with commercially unacceptable toluene conversions of only about 10%, resulting in a yield of not greater than about 9%, i.e., 10% ×90%. Such processes also produce significant quantities of ortho-xylene and meta-xylene thereby necessitating expensive separation processes in order to separate the para-xylene from the other isomers.

Typical separation procedures comprise costly fractional crystallization and adsorptive separation of para-xylene from other xylene isomers which are customarily recycled. Xylene isomerization units are then required for additional conversion of the recycled xylene isomers into an equilibrium mixture comprising para-xylene.

Those skilled in the art appreciate that the expense of the separation process is proportional to the degree of separation required. Therefore, significant cost savings are achieved by increasing selectivity to the para-isomer while maintaining commercially acceptable conversion levels.

The activity of a zeolite is an important consideration in acid-type catalysis such as toluene disproportionation. Silicious zeolites may be considered to contain SiO₄ -tetrahedra. Substitution for the tetravalent element by a trivalent element such as aluminum produces a negative charge which must be balanced. If this is done by a proton, the material is acidic and active. The activity of zeolite catalysts has been described in terms of its Alpha Value.

Various methods have been devised for enhancing the catalytic activity, i.e., the alpha of zeolite materials. U.S. Pat. No. 4,871,702 describes ammonia activation of zeolites by contacting a zeolite and a solid source of aluminum with an aqueous ammonium solution under ammonia gas pressure. U.S. Pat. No. 4,665,248 describes treating a mixture of zeolite and a solid source of aluminum with liquid water in the presence of alkali metal cation. U.S. Pat. No. 4,568,787 discloses contacting a high silica zeolite with aluminum chloride vapor and hydrolyzing. U.S. Pat. No. 4,576,805 describes enhancing catalytic activity of a zeolite by contacting with a volatile metal halide compound to incorporate the metal into the crystalline lattice of the zeolite.

A method for simultaneous silicon modification for selectivity and activity enhancement of a catalyst would have important application in shape-selective catalysis.

Accordingly, it would be highly advantageous to provide a shape selective hydrocarbon conversion process such as toluene disproportionation over a shape selective, highly active catalyst.

SUMMARY OF THE INVENTION

The invention is a process for shape selective hydrocarbon conversions such as the regioselective production of para-xylene. A reaction stream containing hydrocarbon feed such as toluene is contacted under hydrocarbon conversion conditions with a molecular sieve catalyst which has been concurrently pre-selectivated and activated by treating with a composition which includes a substantially aqueous solution of a water soluble organosilicon compound as a first silicon source. In toluene disproportionation, the reaction stream may also contain a second silicon source. These reaction conditions in toluene disproportionation are suitable to provide a single pass para-xylene product, relative to all C₈ products, of at least about 90% and at least 15% toluene conversion.

The invention is also a method for modifying a molecular sieve catalyst having a Constraint Index of 1-12 by treating with a composition which includes a substantially aqueous solution of water soluble organosilicon compound as a first silicon source, and then calcining. The catalyst may be subsequently contacted with a mixture of a second silicon source which is a high-efficiency, para-xylene selectivating agent and toluene at reaction conditions for converting toluene to xylene. The molecular sieve thus treated has greatly enhanced activity and selectivity. The invention also includes the modified catalyst.

Advantageously, a highly selective and highly active catalyst can be provided in the modification method.

Furthermore, in the modification method, the use of aqueous solution of silicon sources is more cost efficient relative to organic solvent and aqueous emulsion procedures. The potentially hazardous distillation and condensation of organic solvents is avoided. Furthermore, improved deposition uniformity is provided using aqueous solutions compared to aqueous emulsions.

In addition, a shape selective hydrocarbon conversion process over the modified catalyst shows enhanced product selectivity with good conversion.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is useful in shape selective hydrocarbon conversion processes such as in converting various aromatics, particularly toluene to commercially useful para-substituted benzenes, particularly para-xylene.

Molecular sieves to be used in the process of the invention include intermediate pore zeolites. Such medium pore zeolites are considered to have a Constraint Index from about 1 to about 12. The method by which Constant Index is determined is described fully in U.S. Pat. No. 4,016,218, incorporated herein by reference. Molecular sieves which conform to the specified values of Constraint Index for intermediate pore zeolites include ZSM-5, ZSM-11, ZSM-5/ZSM-11 intermediate, ZSM-12, ZSM-21, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, ZSM-50, MCM-22 and Zeolite Beta which are described, for example, in U.S. Pat. Nos. 3,702,886 and Re. No. 29,949, 3,709,979, 3,832,449, 4,046,859, 4,556,447, 4,076,842, 4,016,245, 4,229,424, 4,397,827, 4,954,325, 3,308,069, Re. 28,341 and EP 127,399 to which reference is made for details of these molecular sieves. These zeolites may be produced with differing silica:alumina ratios ranging from 12:1 upwards. They have been, in fact, be produced from reaction mixtures from which aluminum is intentionally excluded, so as to produce materials having extremely high silica:alumina ratios which, in theory at least may extend up to infinity. Preferred intermediate pore zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-35 and MCM-22. Particularly preferred is ZSM-5.

In the invention, the catalyst preferably has a silica-alumina ratio less than 100 preferably about 20-80 and an alpha value greater than 100, for example about 150-2000.

The Alpha Value is an approximate indication of the catalytic cracking activity of the catalyst compared to a standard catalyst and it gives the relative rate constant (rate of normal hexane conversion per volume of catalyst per unit time.) It is based on the activity of the amorphous silica-alumina cracking catalyst taken as an Alpha of 1 (Rate Constant=0.016 sec⁻¹). The Alpha Test is described in U.S. Pat. No. 3,354,078 and in The Journal of Catalysis, Vol. 4, pp. 522-529 (August 1965): Vol. 6, p. 278 (1966); and Vol. 61, p. 395 (1980), each incorporated herein by reference as to that description. It is noted that intrinsic rate constants for many acid-catalyzed reactions are proportional to the Alpha Value for a particular crystalline silicate catalyst (see "The Active Site of Acidic Aluminosilicate Catalysts, " Nature, Vol. 309, No . 5959, pp. 589-591, 14 Jun. 1984). The experimental conditions of the test used herein include a constant temperature of 538° C. and a variable flow rate as described in detail in the Journal of Catalysis, Vol. 61, p. 395.

Since toluene disproportionation is an acid-type catalysis and requires a catalyst having inherent acid activity, i.e., an elevated alpha, a silicon modification of the catalyst which increases alpha is highly advantageous. A silicon modification using an aqueous solution of an organosilicon compound has been unexpectedly found to greatly increase alpha while concurrently yielding a catalyst with increased para-selectivity.

The catalyst is modified, i.e., concurrently pre-selectivated and activated, by contact with an aqueous solution of an organosilicon compound. By "pre-selectivation"is meant pretreatment of the catalyst to lower non-shape selective surface acid activity and to increase the rate of subsequent selectivation using para-xylene selectivating agents.

Organosilicon compounds useful herein are water soluble and may be described as organopolysiloxanes. The preferred compounds are polyalkylene oxide modified organopolysiloxanes. The organopolysiloxanes are preferably larger than the pores of the catalyst and do not enter the pores.

The organopolysiloxanes include polysiloxanes having alkyl, alkoxy, aryl, aryloxy, arylalkyl or alkylaryl side groups. The amount of alkylene oxide groups in the organopolysiloxanes can be increased to overcome any hydrophobicity. Alkyl includes 1 to 12 carbons. Aryl includes 6 to 10 carbons. Preferred organopolysiloxanes contained methyl and/or ethyl groups.

Water soluble organosilicon compounds are commercially available as, for example, SAG-5300, manufactured by Union Carbide, conventionally used as an anti-foam, and SF 1188 manufactured by General Electric.

The organosilicon compound is preferably dissolved in an aqueous solution in an organosilicon compound/H₂ O weight ratio of from about 1/100 to about 1/1.

A "solution" is intended to mean a uniformly dispersed mixture of one or more substances at the molecular or ionic level. The skilled artisan will recognize that solutions, both ideal and colloidal, differ from emulsions.

The catalyst is contacted with a substantially aqueous solution of the organosilicon compound at a catalyst/organosilicon compound weight ratio of from about 100 to about 1, at a temperature of about 10° C. to about 150° C., at a pressure of about 0 psig to about 200 psig, for a time of about 0.1 hour to about 24 hours, the water is preferably removed, e.g., by distillation, or evaporation with or without vacuum, and the catalyst is calcined.

Shape Selective Conversions

Zeolites modified in accordance with the invention are generally useful as catalysts in shape selective hydrocarbon conversion processes including cracking reactions involving dewaxing of hydrocarbon feedstocks; isomerization of alkylaromatics; oligomerization of olefins to form gasoline, distillate, lube oils or chemicals; alkylation of aromatics; transalkylation of aromatics; conversion of oxygenates to hydrocarbons; rearrangement of oxygenates; and conversion of light paraffins and olefins to aromatics.

Dewaxing

The subject catalysts have good cracking and hydrocracking activity and may be used to convert paraffins from high to low molecular weight substances in dewaxing processes. The catalysts of the invention may be used in processes such as those described, for example, in U.S. Pat. Nos. 3,700,585, Re. 28,398, 3,968,024 and 4,181,598 which are incorporated herein by references. The term dewaxing means the removal of those hydrocarbons which will readily solidify (waxes) from petroleum stocks. Hydrocarbon feeds which can be treated include lubricating oil stocks as well as those which have a freeze point or pour point problem, i.e., petroleum stocks boiling above 350° F. The dewaxing can be carried out at either cracking or hydrocracking conditions.

In U.S. Pat. No. 3,700,585 and Re. 28,398 to Chen et al., typical cracking conditions include a liquid hourly space velocity (LHSV) between about 0.5 and 200, a temperature between about 288° C. (550° F.) and 590° C. (1100° F.), a pressure between about subatmospheric and several hundred atmospheres over ZSM-5 type catalysts. Typical hydrocracking conditions include a liquid hourly space velocity between about 0.1 and 10, a temperature between about 340° C. (650° F.) and 538° (1000° F.), a pressure between about 100 and 3000 psig, and a hydrogen to hydrocarbon mole ratio between about one and 20. U.S. Pat. No. 3,968,024 describes similar conversions using ZSM-5 of small crystal size. U.S. Pat. No. 4,181,598 describes shape selective cracking to produce lubes.

Isomerization of alkylaromatics

The modified catalysts of the invention are also advantageously used in the isomerization of alkylaromatics in conversion reactions of the type described, for example, in U.S. Pat. Nos. 3,856,872, 3,856,873, Re. 30,157, 4,101,595, 4,101,597, 4,312,790, Re. 31,919 and 4,224,141 which are herein incorporated by reference.

In U.S. Pat. No. 3,856,872 to Morrison, there is described a process for converting C₈ aromatics xylene and ethylbenzene to para-xylene (octafining) at a temperature of 550° F. (288° C.) to 900° F. (482° C.), a pressure of 150 to 300 psig, and a liquid hour space velocity (LHSV) of 1 to 200 over an acid form catalyst containing metal such as platinum or nickel and hydrogen.

In U.S. Pat. No. 3,856,873 to Burress, mixtures of C₈ aromatic hydrocarbons are isomerized to para-xylene by contact in vapor phase with zeolite at a temperature of 500° F. (260° C.) to 1000° F. (538° C.), a pressure of 0 (atmospheric) to 1,000 psig, and a WHSV of 0.5 to 250 with no added hydrogen. The catalyst is an acid ZSM-5, ZSM-12 or ZSM-21.

U.S. Pat. No. 4,101,595 to Chen et al. describes the production of para-xylene from aromatics of 8 to 10 carbons over a dual function catalyst with a shape selective acid catalyzed step at a temperature of 650° F. (343° C.) to 1000° F. (538° C.), a pressure of 50 to 500 psig, a LHSV of 0.1 to 100 and a molar of hydrogen/hydrocarbon of 0.1 to 15. The acid form catalyst has a Constraint Index of 1 to 12, a silica/alumina ratio of at least 12, a crystal density of not less than 1.6 g/cc, may be pre-coked, and includes Group VIII noble metal.

In U.S. Pat. No. 4,101,597 to Breckenridge, a C₈ feed is first isomerized at 550° F. (288° C.) to 700° F. (371° C.) over a zeolite having a Constraint Index of 1 to 12, a silica/alumina ratio of at least 12 and containing a metal having a hydrogenation/dehydrogenation function. A C⁹⁺ fraction produced during isomerization of C₈ is separated from the other isomerization products, blended with hydrogen and toluene and contacted with a porous, acidic catalyst such as ZSM-5 at 750° F. (399° C.) to 900° F. (482° C.). The catalyst has a Constraint Index of 1 to 12, a silica/alumina ratio of at least 12, and a metal providing hydrogenation/dehydrogenation function.

In U.S. Pat. No. 4,224,141 to Morrison, C₈ aromatics are isomerized to benzene, toluene and xylenes over a ZSM-5 which is reduced in activity by dilution with inert matrix, steaming or thermal treatment, very high silica/alumina ratio, base exchange with alkali metal, coking or the like. The conversion is at a temperature of 800° F. (427° C.) to 1000° F. (538° C.) in a low pressure isomerization unit at a pressure only sufficient to overcome pressure drop through downstream processing equipment, e.g. below 100 psig, and a WHSV of 1 to 200.

In U.S. Pat. No. 4,312,790 and Re. 31,919 to Butter et al., a zeolite is incorporated with noble metal subsequent to zeolite crystallization but prior to catalyst extrusion. The catalyst is used for xylene isomerization at a temperature of 500° F. (260° C.) to 1000° F. (540° C.), a pressure between 50 and 1000 psig, a WHSV of 1 to 50 and a hydrogen/hydrocarbon male ratio of 1 to 20.

Conversion of oxygenates to hydrocarbons

U.S. Pat. No. 4,476,330 to Kerr et al., herein incorporated by reference, describes the conversion of aliphatic oxygenates to a higher molecular weight compound by contacting with a zeolite having a silica/alumina ratio substantially greater than 10 at a temperature of 70° F. (21° C.) to 1400° F. (760° C.). The feeds include lower aliphatic organic oxygenates up to C₆, acetals, ketals, acid halides, alcohols, carboxylic acids, aldehydes, acid anhydrides, epoxides, ethers, esters, hemiacetals, gem diols, hydroxy acids, ketones, ketenes, lactones, peracids, peroxides, sugars, and especially alcohols, ethers and esters.

Oligomerization of olefins

The modified catalysts of the invention are advantageously used in the oligomerization of olefins to form gasoline, distillate, lube oils or chemicals in conversion reactions of the type described, for example, in U.S. Pat. Nos. 4,517,399, 4,520,221, 4,547,609 and 4,547,613 which are herein incorporated by reference.

U.S. Pat. No. 4,517,399 to Chester et al. describes the conversion of olefins of 3 to 18 carbons, e.g. propylene, to high viscosity, low pour point lubricating oils by contacting with ZSM-5 type zeolites having large crystals of at least two microns. The conversion conditions include a temperature of 350° F. (177° C.) to 650° F. (343° C.) a pressure of 100 to 5000 psig, and a WHSV of 0.1 to 10.

U.S. Pat. No. 4,520,221 to Chen describes the polymerization of olefins of 2 to 8 carbons, e.g. propylene, butylene, to high viscosity lubes, e.g. linear hydrocarbons, over highly siliceous, acidic ZSM-5 type catalysts with surface acidity inactivated by treatment with base, e.g. bulky amines with a cross-section larger than about 5 Angstroms. The conversion involves a one or two stage process with the polymerization of lower olefins to linear materials, e.g. at about 200° C. over a surface poisoned zeolite, and oligomerization of the product over a modified or unmodified catalyst at a temperature of 50°-75° lower than the first stage, e.g. 150° C. Therefore, the temperatures range from 25° C. to 400° C., with a pressure of atmospheric to 1500 psi and a WHSV of 0.04 to 1.0.

U.S. Pat. No. 4,547,609 to Dessau describes a two stage process whereby in the first stage, light olefins of 2 to 6 carbons are oligomerized to gasoline and distillate liquids including aliphatics of 10 to 20 carbons over a zeolite having a crystal size greater than 0.5 micron at conditions including at a temperature of 500° F. (260° C.) or higher, e.g. a range of 500° F. (260° C.) to 800° F. (437° C.), a pressure of atmospheric to 2000 psig and a WHSV of 0.1 to 20. In the second stage, the distillate fraction is converted to high viscosity lubes by contact with a zeolite of smaller crystal size under milder conditions of a temperature about 200° F. (100° C.) to 500° F. (260° C.), a pressure of atmospheric to 650 psig, and a WHSV less than one.

U.S. Pat. No. 4,547,613 to Garwood et al. describes converting olefins of 2 to 16 carbons to high viscosity lube oil. A ZSM-5 type catalyst is pre-conditioned by contact with light olefins of 2 to 16 carbons, e.g. propylene at 400° F. (204° C.) to 1000° F. (538° C.), at a pressure of 0 to 100 psig for 1 to 70 hours. Conversion conditions include a temperature of 350° F. (177° C.) to 650° F. (343° C.), a pressure of 400 to 5000 psig and a WHSV of 0.1 to 10. The lube fraction may be subjected to a hydrogenation step to stabilize.

Conversion of aromatics to dialkyl-substituted benzene

The modified zeolite catalysts of the invention are advantageously used in the conversion of aromatics compounds to provide dialkyl-substituted benzene products which are highly enriched in the para-dialkyl substituted benzene isomer. Conversion reactions of this type include aromatics alkylation, transalkylation and disproportionation. Aromatics alkylations in which the catalysts of the invention can be used are described, for example, in U.S. Pat. Nos. 3,755,483, 4,086,287, 4,117,024 and 4,117,026 which are herein incorporated by reference.

As described in U.S. Pat. No. 3,755,483 to Burress, aromatic hydrocarbons such as benzenes, naphthalenes, anthracenes and substituted derivatives thereof, e.g. toluene and xylene, may be alkylated with alkylating agents such as olefins ethylene, propylene, dodecene, and formaldehyde, alkyl halides, and alkyl alcohols with 1 to 24 carbons under vapor phase conditions including a reactor inlet temperature up to about 900° F. (482° C.), with a reactor bed temperature up to about 1050° F. (566° ), at a pressure of about atmospheric to about 3000 psig, a ratio of aromatic/alkylating agent of about 1:1 to about 20:1 and a WHSV of 20 to 3000 over ZSM-12.

As described in U.S. Pat. No. 4,086,287 to Kaeding et al., monoalkylbenzenes having alkyls of 1-2 carbons, such as toluene and ethylbenzene, may be ethylated to produce a para-ethyl derivative, e.g. para-ethyltoluene at a temperature of from about 250° C. to about 600° C., a pressure of 0.1 atmospheres to 100 atmospheres, a weight hourly space velocity (WHSV) of 0.1 to 100, and a ratio of feed/ethylating agent of 1 to 10 over a catalyst having an acid activity, i.e., alpha, of 2 to 5000, modified by pre-coking or combining with oxides of phosphorus, boron or antimony to attain a catalyst with a xylene sorption capacity greater than 1 g/100 g of zeolite and an ortho-xylene sorption time for 30% of said capacity of greater than 10 minutes, where sorption capacity and sorption time are measured at 120° C. and a xylene pressure of 4.5±0.8 mm of mercury.

U.S. Pat. No. 4,117,024 to Kaeding describes a process for the ethylation of toluene or ethylbenzene to produce p-ethyltoluene at a temperature of 350° C. to 550° C., a critical pressure of greater than one atmosphere and less than 400 psig, with hydrogen/ethylene ratio of 0.5 to 10 to reduce aging of the catalyst. The zeolite described in U.S. Pat. No. 4,117,024 has a crystal size greater then one micron, and is modified as the catalyst in U.S. Pat. No. 4,086,287 to attain the sorption capacity described in U.S. Pat. No. 4,086,287.

U.S. Pat. No. 4,117,026 to Haag and Olson describes the production of para-dialkyl benzenes having alkyls of 1 to 4 carbons under conditions which vary according to the feed. When the feed includes monoalkyl substituted benzenes having an alkyl of 1 to 4 carbons, olefins of 2 to 15, or paraffins of 3 to 60 carbons or mixtures thereof, conversion conditions include a temperature of 250° C. to 750°, a pressure of 0.1 to 100 atmospheres and a WHSV of 0.1 to 2000. For the disproportionation of toluene, the conditions include a temperature of 400° C. to 700° C., a pressure of 1 to 100 atmospheres and a WHSV of 1-50. When the feed includes olefins of 2 to 15 carbons including cyclic olefins, the conversion conditions include a temperature of 300° C. to 700° C., a pressure of 1 to 100 atmospheres and a WHSV of 1 to 1000. When the feed includes paraffins of 3 to 60 carbons, conditions include a temperature of 300° C. to 700° C., a pressure of 1 to 100 atmospheres and a WHSV of 0.1 to 100. However for lower paraffins of 3 to 5 carbons, the temperature should be above 400° C. When the feed includes mixed aromatics such as ethylbenzene and toluene, and also optionally olefins of 2 to 20 carbons or paraffins of 5 to 25 carbons, conversion conditions includes a temperature of 250° C. to 500° C. and a pressure greater than 200 psig. In the absence of added aromatics, the olefins and higher paraffins are more reactive and require lower severity of operation, e.g. a temperature of 250° C. to 600° C., preferably 300°-550° C. The catalyst described in U.S. Pat. No. 4,117,026 is modified as in U.S. Pat. No. 4,086,287.

Conversion of light paraffins and olefins to aromatics

The modified catalysts of the invention may also be used in the conversion of light paraffins and olefins to aromatics in processes of the type described, for example, in U.S. Pat. Nos. 3,760,024 and 3,756,942 which are herein incorporated by reference.

U.S. Pat. No. 3,760,024 to Cattanach describes a process for the conversion of paraffins of 2 to 4 carbons and/or olefins to aromatics of 6 to 10 carbons over a ZSM-5 type catalyst with or without hydrogenation/dehydrogenation component. Conversion conditions include a temperature of 100° C. to 650° C., a pressure of 0 to 1,000 psig, a WHSV of 0.1 to 500 and a hydrogen/hydrocarbon ratio of 0 to 20.

U.S. Pat. No. 3,756,942 to Cattanach describes the conversion of paraffins, olefins and naphthenes to aromatics over ZSM-5 type catalysts. If the feed contains at least 35 wt. % olefins, conversion is at 650° F. (363° C.) to 1400° F. (760° C.). If the feed contains less than 35 wt. % olefins, the temperature is 900° F. (482° C.) to 1400° F. (760° C.) with the absence of substantial added hydrogen. For both types of feed, the pressure is atmospheric to 35 atmospheres and the WHSV 1 to 15.

Pyridine synthesis

The modified catalysts of the invention are also advantageously used in the synthesis of pyridine. Pyridine bases may be produced through the reactions of aldehydes and ketones with ammonia. The reaction of acetaldehyde with ammonia in the presence of heterogenous catalysts at about 350° C. to about 550° C. yields 2- and 4-methylpyridine. Acetaldehyde, formaldehyde and ammonia react to yield pyridine and 3-methylpyridine. Pyridine synthesis is described, for example, in U.S. Pat. No. 4,675,410 to Feitler and U.S. Pat. No. 4,220,783 to Chang et al. which are herein incorporated by reference.

Caprolactam synthesis

Caprolactam is used in the commercial production of nylon. Caprolactam may be produced by Beckmann rearrangement of cyclohexane oxime over acid catalysts including zeolites. The synthesis of caprolactam is described, for example, in U.S. Pat. No. 4,359,421 which is herein incorporated by reference.

Therefore, the modified catalysts of the present invention are suitable for use in a variety of shape selective hydrocarbon conversion processes including as non-limiting examples, cracking hydrocarbons with reaction conditions including a temperature of from about 300° C. to about 700° C., a pressure of from about 0.1 atmosphere (bar) to about 30 atmospheres and a weight hourly space velocity of from about 0.1 hr⁻¹ to about 20 hr⁻ ; dehydrogenating hydrocarbon compounds with reaction conditions including a temperature of from about 300° C. to about 700° C., a pressure of from about 0.1 atmosphere to about 10 atmospheres and weight hourly space velocity of from about 0.1 to about 20; converting paraffins to aromatics with reaction conditions including a temperature of from about 300° C. to about 700° C., a pressure of from about 0.1 atmosphere to about 60 atmospheres, a weight hourly space velocity of from about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio of from about 0 to about 20; converting olefins to aromatics, e.g. benzene, toluene and xylene, with reaction conditions including a temperature of from about 100° C. to about 700° C., a pressure of from about 0.1 atmosphere to about 60 atmospheres, a weight hourly space velocity of from about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio of from about 0 to about 20; converting alcohols, e.g. methanol, or ethers, e.g. dimethylether, or mixtures thereof to hydrocarbons including olefins and/or aromatics with reaction conditions including a temperature of from about 275° C. to about 600° C., a pressure of from about 0.5 atmosphere to about 50 atmospheres and a liquid hourly space velocity of from about 0.5 to about 100; isomerizing xylene feedstock components with reaction conditions including a temperature of from about 230° C. to about 510° C., a pressure of from about 3 atmospheres to about 35 atmospheres, a weight hourly space velocity of from about 0.1 to about 200 and a hydrogen/hydrocarbon mole ratio of from about 0 to about 100; disproportionating toluene with reaction conditions including a temperature of from about 200° C. to about 760° C., a pressure from about atmospheric to about 60 atmospheres and a weight hourly space velocity of from about 0.08 to about 20; alkylating aromatic hydrocarbons, e.g. benzene and alkylbenzenes in the presence of an alkylating agent, e.g. olefins, formaldehyde, alkyl halides and alcohols, with reaction conditions including a temperature of from about 250° C. to about 500° C., a pressure of from about atmospheric to about 200 atmospheres, a weight hourly space velocity of from about 2 to about 2000 and an aromatic hydrocarbon/alkylating agent mole ratio of from about 1/1 to about 20/1; and transalkylkating aromatic hydrocarbons in the presence of polyalkylaromatic hydrocarbons with reaction conditions including a temperature of from about 340° C. to about 500° C., a pressure of from about atmospheric to about 200 atmospheres, a weight hourly space velocity of from about 10 to about 1000 and an aromatic hydrocarbon/polyalkylaromatic hydrocarbon mole ratio of from about 1/1 to about 16/1.

In general, therefore, catalytic conversion conditions over a catalyst comprising the modified zeolite include a temperature of from about 100° C. to about 760° C., a pressure of from about 0.1 atmosphere (bar) to about 200 atmospheres (bar), a weight hourly space velocity of from about 0.08 hr⁻¹ to about 2000 hr⁻¹ and a hydrogen/organic, e.g. hydrocarbon compound of from 0 to about 100.

Toluene Disproportionation

Toluene Disproportionation will be used as a representative shape selective conversion. A catalyst treated in the manner described herein yields a para-selective product in toluene disproportionation. Reaction conditions in the disproportionation include temperatures ranging from about 100° C. to about 600° C., preferably from about 300° C. to about 500° C.; pressures ranging from about 0 to about 2000 psig, preferably from about 15 to about 800 psig; a mole ratio of hydrogen to hydrocarbons from about 0 (i.e. no hydrogen is present) to about 10, preferably from about 1 to about 4; at a weight hourly space velocity (WHSV) from about 0.1 to about 100 hr⁻¹, preferably from about 0.1 to about 10 hr⁻¹.

Normally a single pass conversion of a toluene stream results in a product stream which includes dimethylbenzenes having alkyl groups at all locations, i.e., ortho-, meta-, and para-xylenes. Furthermore, the xylenes are known to proceed in a reaction which produces unwanted ethylbenzenes (EB) by the following reaction: ##STR2##

Previously, the purity of p-xylene with respect to all of the C₈ products in a single pass has been limited to less than 90% when isomerization is permitted. This efficiency is reduced somewhat by the production of ethylbenzene.

The present invention, however, provides high efficiency conversion which reduces production of ortho- and meta-isomers to the benefit of the desired para-isomer. The resulting product stream contains greater than a 90% purity of para-xylene. For example, the ortho-xylene isomer can be reduced to not more than about 0.5% of the total xylenes content while the meta-xylene isomer can be reduced to less than about 5% of the total xylene content. Moreover, when the reaction system is properly treated, such as by deposition of platinum on the molecular sieve, the presence of ethylbenzene can be reduced to less than about 0.3% of the C₈ product.

As explained in greater detail herein, the present invention provides a method for obtaining para-xylene at conversion rates of at least about 15%, preferably at least about 20-25%, and with para-xylene purity of greater than 90%, preferably at least 95%, and most preferably about 99%.

Therefore, higher para-xylene purity at commercially acceptable conversion rates than previously disclosed processes. The present invention thus allows for a significant reduction in process costs previously associated with the separation of unwanted by-products. Processes of the prior art typically require expensive secondary and tertiary treatment procedures in order to obtain these efficiencies.

The present invention includes the regioselective conversion of toluene to para-xylene by methylating toluene in a reaction stream containing a toluene feed with a trim selectivated catalytic molecular sieve which has been pre-selectivated and activated, with conversion reaction conditions to provide a single pass, para-xylene purity of at least about 90% based on the C₈ products. The trim selectivation methods are described below. As used herein, the term "para-xylene purity" means the percentage of para-xylene in all of the C₈ products which include ethylbenzene, para-xylene, ortho-xylene, and meta-xylene. Those skilled in the art will appreciate that the proximity of the boiling points of these C₈ products necessitates more expensive separation processes whereas para-xylene may be more readily separated from other components in the product stream such as benzene, toluene, and para-ethyltoluene.

As used herein, the term "xylene-conversion product" indicates the total amount of xylenes resulting from the disproportionation reaction. The word "para-xylene" in this term is not intended to limit the scope of the present invention to the production of xylenes since other para-substituted aromatics may be produced.

In a preferred embodiment, the invention also includes a method for the regioselective production of para-xylene by passing a reaction stream which contains an aromatic feedstock, e.g., toluene, in a single pass, over a trim-selectivated catalytic molecular sieve, which is pre-selectivated, the single pass in the presence of hydrogen at reaction conditions suitable to provide para-xylene purity of greater than about 90%. The product stream may also include small amounts of ortho- and meta-xylene and trace amounts of impurities such as ethylbenzene.

The toluene may be fed simultaneously with a high-efficiency selectivating agent and hydrogen at reaction conditions until the desired p-xylene selectivity, e.g., 90% or 95%, is attained, whereupon the feed of selectivating agent is discontinued. This co-feeding of selectivating agent with toluene will be termed "trim selectivation". Reaction conditions for this trim-selectivation step generally include a temperature of about 350°-540° C. and a pressure of about atmospheric--5000 psig. The feed is provided to the system at a rate of about 0.1-20 WHSV. The hydrogen is fed at a hydrogen to hydrocarbon molar ratio of about 0.1-20.

The high efficiency para-xylene selectivating agent for trim selectivation preferably comprises a silicon containing compound discussed in greater detail below. For example, organic silicons such as phenylmethyl silicone, dimethyl silicone, and mixtures thereof are suitable. According to one embodiment of the present invention, a silicone containing phenylmethylsilicon and dimethylsilicon groups in a ratio of about 1:1 is co-fed to the system, while the other components, e.g., toluene and hydrogen, are fed in the amounts set forth above. The high-efficiency para-xylene selectivating agent is fed in an amount of about 0.1% -50% of the toluene according to this preferred embodiment. Depending upon the percentage of selectivating agent used, the trim selectivation will preferably last for about 50-300 hours, most preferably less than 170 hrs.

The catalyst is pre-selectivated ex situ with a water soluble organosilicon compound, then calcined and subsequently may be trim selectivated with a high efficiency para-xylene selectivating agent. After pre-selectivation, the catalytic molecular sieves for the present invention are preferably converted to the hydrogen form. The crystal size of zeolites used herein is preferably greater than 0.1 micron.

As used herein, the term "high efficiency, p-xylene selectivating agent" as used for trim selectivation is used to indicate substances which will increase the para-selectivity of a catalytic molecular sieve to the stated levels while maintaining commercially acceptable toluene to xylene conversion levels. Such substances include, for example, organic silicon compounds such as phenylmethyl silicone, dimethylsilicone, and blends thereof which have been found to be suitable.

The trim selectivation of the catalyst is preferably performed with a silicone containing compound. An example of silicone compounds which can be used in the present invention can be characterized by the general formula: ##STR3## where R₁ is hydrogen, fluorine, hydroxy, alkyl, aralkyl, alkaryl or fluoro-alkyl. The hydrocarbon substituents generally contain from 1 to 10 carbon atoms and preferably are methyl or ethyl groups. R₂ is selected from the same group as R₁, and n is an integer of at least 2 and generally in the range of 3 to 1000. The molecular weight of the silicone compound employed is generally between about 80 and about 20,000 and preferably within the approximate range of 150 to 10,000. Representative silicone compounds include dimethylsilicone, diethylsilicone, phenylmethylsilicone, methylhydrogensilicone, ethylhydrogensilicone, phenylhydrogensilicone, methylethylsilicone, phenylethylsilicone, diphenylsilicone, methyltrifluoropropylsilicone, ethyltrifluoropropysilicone, polydimethylsilicone, tetrachlorophenylmethyl silicone, tetrachlorophenylethyl silicone, tetrachlorophenylhydrogen silicone, tetrachlorophenylphenyl silicone, methylvinylsilicone and ethylvinylsilicone. The silicone compound need not be linear but may be cyclic as for example hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, hexaphenylcyclotrisiloxane and octaphenylcyclotetrasiloxane. Mixtures of these compounds may also be used as well as silicones with other functional groups. Other silicon-containing compounds, such as silanes and siloxanes, may also be utilized.

Preferably, the kinetic diameter of the high efficiency, p-xylene selectivating agent is larger than the zeolite pore diameter, in order to avoid reducing the internal activity of the catalyst.

Before trim-selectivation, the catalyst is pre-selectivated, and a silicon compound is deposited on the external surface of the catalyst.

Following deposition of the silicon-containing compound in pre-selectivation, the catalyst is calcined. For example, the catalyst may be calcined in an oxygen-containing atmosphere, preferably air, at a rate of 0.2° to 5° C./minute to a temperature greater 300° C. but below a temperature at which the crystallinity of the zeolite is adversely affected. Generally, such temperature will be below 600° C. Preferably the temperature of calcination is within the approximate range of 350° to 550° C. The product is maintained at the calcination temperature usually for 1 to 24 hours.

While not wishing to be bound by theory, it is believed that the advantages of the present invention are obtained, in part, by rendering acid sites on the external surfaces of the catalyst substantially inaccessible to reactants while increasing catalyst tortuosity. Acid sites existing on the external surface of the catalyst are believed to isomerize the para-xylene exiting the catalyst pores back to an equilibrium level with the other two isomers thereby reducing the amount of para-xylene in the xylenes to only about 24%. By reducing the availability of these acid sites to the para-xylene exiting the pores of the catalyst, the relatively high level of para-xylene can be maintained. It is believed that the high-efficiency, p-xylene selectivity agents of the present invention block or otherwise render these external acid sites unavailable to the para-xylene by chemically modifying said sites.

In line with this theory, it is also believed that the presence of hydrogen in the reaction zone during the trim selectivation is important in order to maintain the desired high yields of para-xylene when a silicone compound is used as the high-efficiency para-xylene selectivating agent. The importance of the hydrogen may be reduced in alternative embodiments by using a high efficiency para-xylene selectivating agent comprising silane or some other compound which effectively renders the isomerizing acid sites on the external surface of the catalyst inaccessible.

The invention may utilize a high efficiency para-xylene selectivating agent which includes a silicon compound wherein the silicon compound is introduced by co-feeding, for example, at least one silicon compound with the toluene feedstock over a conversion catalyst at reaction conditions until the desired degree of selectivation is achieved, at which time the feed of selectivating agent may be discontinued.

The toluene feedstock preferably includes about 50% to 100% toluene, more preferably at least about 80% toluene in the toluene feedstock. Other compounds such as benzene, xylenes, and trimethylbenzene may also be present in the toluene feedstock without adversely affecting the present invention.

The toluene feedstock may also be dried, if desired, in a manner which will minimize moisture entering the reaction zone. Methods known in the art suitable for drying the toluene charge for the present process are numerous. These methods include percolation through any suitable desiccant, for example, silica gel, activated alumina, molecular sieves or other suitable substances, or the use of liquid charge dryers.

For the improved disproportionation process of this invention, the suitable molecular sieve may be employed in combination with a support or binder material such as, for example, a porous inorganic oxide support or a clay binder. While the preferred binder is silica, other non-limiting examples of such binder materials include alumina, zirconia, magnesia, thoria, titanic, boria and combinations thereof, generally in the form of dried inorganic oxide gels or gelatinous precipitates. Suitable clay materials include, by way of example, bentonite and kieselguhr. The relative proportion of suitable crystalline molecular sieve to the total composition of catalyst and binder or support may be about 30 to about 90 percent by weight and is preferably about 50-80 percent by weight of the composition. The composition may be in the form of an extrudate, beads or fluidizable microspheres.

Operating conditions employed in the improved process of the present invention may be adjusted to affect the para-selectivity and toluene conversion rate. Such conditions include the temperature, pressure, space velocity, molar ratio of the reactants, and the hydrogen to hydrocarbon mole ratio. One preferred embodiment of the present invention includes contacting a catalytic molecular sieve with a toluene feedstock which includes a silicone compound under conditions for effecting vapor-phase disproportionation. Conditions effective for accomplishing the high para-selectivity and acceptable toluene disproportionation conversion rates include a reactor inlet temperature of about 350°-540° C., preferably greater than about 400° C., a pressure of about atmospheric--5000 psig, preferably about 100 to 1000 psig, a WHSV of about 0.1-20, preferably about 2-4, and a hydrogen to hydrocarbon mole ratio of about 0.1-20, preferably about 2-4. This process may be conducted in either batch or fluid bed operation with attendant benefits of either operation readily obtainable.

The effluent is separated and distilled to remove the desired product, i.e., para-xylene, plus other by-products.

The catalyst may be further modified in order to reduce the amount of undesirable by-products, particularly ethylbenzene. The state of the art is such that the reactor effluent from standard toluene disproportionation typically contains about 0.5% ethylbenzene by-product. Upon distillation of the reaction products, the level of ethylbenzene in the C₈ fraction often increases to about 3-4 percent. This level of ethylbenzene is unacceptable for polymer grade p-xylene since ethylbenzene in the C₈ product, if not removed, degrades the quality of fibers ultimately produced from the p-xylene product. Consequently, ethylbenzene content must be kept low. The specification for ethylbenzene in the C₈ product has been determined by industry to be less than 0.3%. Ethylbenzene can be substantially removed by isomerization or by superfractionation processes. Removal of the ethylbenzene by conventional isomerization would be impractical with the present invention since the xylene stream, which includes greater than 90% para-xylene, would be concurrently isomerized to equilibrium xylenes reducing the amount of para-xylene in this xylene stream to about 24%. It is known in the art that the alternative procedure of removing the ethylbenzene by superfractionation is extremely expensive.

In order to avoid the need for downstream ethylbenzene removal, the level of ethylbenzene by-product is advantageously reduced by incorporating a hydrogenation-dehydrogenation function in the catalyst, such as by addition of a metal compound such as platinum. While platinum is the preferred metal, other metals such as palladium, nickel, copper, cobalt, molybdenum, rhodium, ruthenium, silver, gold, mercury, osmium, iron, zinc, cadmium, and mixtures thereof may be utilized. The metal may be added by cation exchange, in amounts of about 0.01-2%, typically about 0.5%. The metal must be able to enter the pores of the catalyst in order to survive a subsequent calcination step. For example, a platinum modified catalyst can be prepared by first adding the catalyst to a solution of ammonium nitrate in order to convert the catalyst to the ammonium form. The catalyst is subsequently contacted with an aqueous solution of tetraamine platinum(II) nitrate or tetraamine platinum(II) chloride. The metallic compound advantageously enters the pores of the catalyst. The catalyst can then be filtered, washed with water and calcined at temperatures of about 250° to 500° C. The hydrogenation-dehydrogenation metal ions are preferably introduced into the catalyst before pre-selectivation.

By the present process, toluene can be converted to aromatic concentrates of high value, e.g., about 99% para-xylene based on all C₈ products. In a typical embodiment of the present process, optimum toluene conversion is found to be about 20-25 weight percent with a para-xylene purity of about 90-99%.

The following non-limiting examples illustrate the invention:

EXAMPLE 1

Silica-modified HZSM-5 was prepared by adding 2.5 g silica bound HZSM-5 to 0.29 g organopolysiloxane, a dimethyl silicone fluid modified to render it water soluble (SAG-5300, Union Carbide). The water was distilled off and the residue was air calcined at 2° C. per minute to 538° C., then six hours at 538° C. The silica-modified HZSM-5 product contained 8.5% added silica.

The alpha value of the silica-modified catalyst was 1622 compared to 731 for the parent zeolite.

COMPARATIVE EXAMPLES

Silica-modified HZSM-5 catalysts having similar silica loading and prepared by aqueous emulsion and organic solvent procedures decreased the alpha value of HZSM-5 from 180 to 52 and from 199 to 67 respectively.

EXAMPLE 2

The catalyst prepared in Example 1 was tested for toluene disproportionation at 400° C., 4.0 WHSV, 500 psig and a 2/1 hydrogen/hydrocarbon ratio. Conversion was 28% indicating a very active catalyst. Xylene equilibrium was 22% p-, 55% m- and 23% o-.

EXAMPLE 3

The catalyst prepared in Example 1 was trim selectivated using 1% phenylmethyl silicone in toluene feed at 446° C., 500 psig, 4.0 WHSV, and hydrogen/hydrocarbon ratio =2. The following table shows toluene conversion and p-xylene selectivity as a function of time on stream. The trim selectivation substantially increased p-xylene selectivity from 24% to 90% at a commercially attractive 23% toluene conversion.

                  TABLE 1                                                          ______________________________________                                         Time on     Toluene        p-Xylene in                                         Stream, Hrs.                                                                               Conversion, 10 + %                                                                            Xylenes, Wt %                                       ______________________________________                                          2          51             24                                                   6          51             26                                                  24          40             47                                                  48          35             68                                                  72          32             78                                                  96          29             83                                                  165         23             90                                                  ______________________________________                                     

What is claimed is:
 1. A method for concurrently pre-selectivating and activating a molecular sieve catalyst comprising treating the catalyst with a composition which comprises an aqueous solution of a water soluble organosilicon compound in an organosilicon compound/H₂ O weight ratio of from about 1/100 to about 1/1, at a molecular sieve/organosilicon compound weight ratio of from about 100/1 to about 1/1, at a temperature of from about 10° C. to about 150° C., at a pressure of from about 0 psig to about 200 psig, for a time of about 0.1 hours to about 24 hours to provide a pre-selectivated, activated molecular sieve catalyst.
 2. The method of claim 1 wherein the molecular sieve catalyst comprises molecular sieve crystals and matrix material.
 3. The method of claim 2 wherein the matrix material comprises silica.
 4. The method of claim 2 wherein the molecular sieve catalyst is calcined after being treated.
 5. The method of claim 1 wherein the organosilicon compound comprises a water soluble polyalkylene oxide modified polysiloxane.
 6. The method of claim 5 wherein the polysiloxane is polyethylene oxide modified.
 7. The method of claim 1 further comprising contacting a pre-selectivated and activated molecular sieve catalyst with a mixture comprising toluene and a second silicon source which is a high efficiency, para-xylene selectivating agent at reaction conditions for converting toluene to xylene for at least one hour to yield a twice modified catalyst.
 8. The method of claim 7 wherein the second silicon source is a silicone.
 9. The method of claim 8 wherein the silicone comprises a mixture of phenylmethylsilicone and dimethylsilicone.
 10. The molecular sieve catalyst product of claim
 1. 11. The molecular sieve catalyst product of claim
 7. 12. A catalyst comprising a molecular sieve having a Constraint Index of about 1-12 which has been concurrently pre-selectivated and activated by treating with an aqueous solution of a water soluble organosilicon compound.
 13. The catalyst of claim 12 wherein the molecular sieve is ion exchanged to contain ions selected from a group consisting of hydrogen, hydrogen precursor, metals of Periodic Table Group VIII and combinations thereof.
 14. The catalyst of claim 11 wherein the catalyst further comprises a matrix material.
 15. The catalyst of claim 12 wherein the matrix material comprises silica.
 16. The catalyst of claim 12 wherein the treating is with a composition which comprises an aqueous solution of a water soluble organosilicon compound in an organosilicon compound/H₂ O weight ratio of from about 1/100 to about 1/1, at a molecular sieve/organosilicon compound weight ratio of from about 100/1 to about 1/1, at a temperature of from about 10° C. to about 150° C., at a pressure of from about 0 psig to about 200 psig, for a time of about 0.1 hours to about 24 hours.
 17. The catalyst of claim 12 wherein the organosilicon compound comprises a water soluble polyalkylene oxide modified polysiloxane.
 18. The catalyst of claim 17 wherein the polysiloxane is polyethylene oxide modified.
 19. The catalyst of claim 12 which has been further treated by contact with a mixture comprising toluene and a second silicon source which is a high efficiency, para-xylene selectivating agent at reaction conditions for converting toluene to xylene for at least one hour to yield a twice modified catalyst.
 20. The catalyst of claim 19 wherein the high efficiency para-xylene selectivating agent comprises a silicone compound.
 21. The catalyst of claim 20 wherein the silicone compound comprises a mixture of phenylmethylsilicone and dimethylsilicone. 